Historical formulations of lake pigments and dyes derived from lac: A study of compositional variability

“Lac” is a term referring to a pink-red-purple organic colourant derived from an insect and used as a lake pigment or a dye. Although historical sources indicate extensive usage, findings in historical samples remain scarce and are based on the detection of laccaic acids A and B, which are the main colourant molecules in lac. In this work various lake pigment and dye production methods were investigated, with the aim to explore the compositional variability derived from changing fundamental parameters, such as temperature and pH. Exhaustive research focusing on historical recipes was fundamental in order to create a series of 16 mock-up samples. Preliminary investigations were carried out using colorimetry and photo-induced luminescence imaging to evaluate the differences in colour and luminescence behaviour. Visible-induced visible luminescence imaging (VIVL) in the range c. 540–700nm was particularly efficient at detecting the presence of included shellac in lac lake pigments. High performance liquid chromatography coupled with an online diode array detector followed by electrospray ionisation and quadrupole time-of-flight detection (HPLC-DAD-ESI-Q-ToF) was ultimately applied to assess the chemical composition of the samples and principle component analysis (PCA) was used to establish correlations between the chemical composition and the production methods. The results demonstrated that, regardless of the pigment/dye production method, a transfer of shellac components always occurs from the sticklac raw material into the lake pigment or dye. More specifically, although an increase in temperature and alkaline pH during the extraction promote the inclusion of high molecular weight shellac components, a variety of shellac components are also found when mild conditions, such as room temperature and neutral pH, are used. The results obtained for artificially aged lac-dyed textile samples showed that lac colourants are more prone to degradation than shellac components. The use of mass spectrometric detection for the identification of lac in historical objects is therefore recommended, as this technique is able to detect non-colourant molecules, which may be present in the pigment or dye as a result of the production methods

Mapping degradation pathways of natural and synthetic dyes with LC-MS: Influence of solvent on degradation mechanisms

To help conserve the vast array of (combinations of) dyestuffs and pigments encountered in cultural-heritage objects and application materials, a rapid and convenient method for dye-degradation research is required. In-solution degradation studies of dyes in a strong solvent, such as the commonly used dimethyl sulfoxide (DMSO), are potentially of interest, in addition to studies involving other solvents, such as water. The degradation of eosin and carminic acid under the influence of light was investigated in two solvents, i.e. in a mixture of DMSO and acetonitrile and in pure water. A liquid chromatography - mass spectrometry (LC-MS) method was developed for analysis of the degraded samples and identification of the individual components. The presence of DMSO generally facilitated faster degradation, which, in combination with its universal solvating properties are advantageous. However, different products were formed in the presence of DMSO. Degradation pathways for eosin and carminic acid in these solvents are proposed

Characterization of Dye Extracts from Historical Cultural-Heritage Objects using State-of-the-Art Comprehensive Two-Dimensional Liquid Chromatography and Mass Spectrometry with Active Modulation and Optimized Shifting Gradients

Unbiased characterization of dyes and their degradation products in cultural-heritage objects requires an analytical method which provides universal separation power regardless of dye classes. Dyes are small molecules that vary widely in chemical structure and properties, which renders their characterization by a single method challenging. We have developed a comprehensive two-dimensional liquid chromatography method hyphenated with mass spectrometry and UV–vis detection. We use stationary-phase-assisted modulation to enhance the method in terms of detection limits and solvent compatibility and to reduce the analysis time. The PIOTR program was used to optimize an assembly of shifting second-dimension gradients, which resulted in a high degree of orthogonality (80% in terms of the asterisk concept). The resulting method is universally applicable to all classes of dyes extracted from cultural-heritage objects. Thanks to the high peak capacity and orthogonality, dye components can be separated from chemically similar impurities and degradation products, providing a detailed fingerprint of the dyes mixture in a specific sample. The method was applied to a number of challenging dye extracts from 17th- and 19th-century cultural-heritage objects